When you start a new thread on a desktop or laptop, you are in advanced mode. If you are replying to an post, you can get to advanced mode by clicking the go advanced button under the input box. In advanced mode, you can see a paperclip icon. If you click it, a window will pop up to allow you to upload pictures when they are uploaded you can insert inline. You can also click and drag previously uploaded pictures to the attachments section.
Note 1 - If you are on a phone or tablet, put it down and find a desktop or laptop.
Note 2 - If you are on tapatalk, you must be on a phone or a tablet. See Note 1.
If you want to use some other storage such as imgur (which isn't too bad in my opinion), try to find one that gives you links in various formats. Find the BBCode link to copy and paste into your post.